The choroid plexus is, amongst other things, a filter for cerebrospinal fluid – you might think of this role as analogous to that of the kidney as a filter for blood, though the two organs are very different in structure at every level, and the choroid plexus also produces the fluid it filters. Like all of the systems in the body and brain, the choroid plexus progressively fails in its function with age, and researchers have reason to believe that this failure contributes to conditions such as Alzheimer’s disease:

An organ in the brain called the choroid plexus apparently plays a critical role in preventing the accumulation of a protein associated with Alzheimer’s disease. The researchers found that the choroid plexus acts as a sort of ‘fishnet’ that captures the protein, called beta-amyloid, and prevents it from building up in the cerebrospinal fluid, which surrounds and bathes the brain and spinal cord. Moreover, tissue in the organ is able to soak up large amounts of the protein and may contain enzymes capable of digesting beta-amyloid.

Levels of beta-amyloid in the brain are more dynamic than their slow buildup over the years implies. You might think of the condition – and indeed the increase in amyloid levels in aging in general – as a slowly progressing imbalance of amyloid creation and clearance mechanisms rather than a slow and irrevocable deposition of amyloid. That in turn implies that a working therapy could quickly reverse all but the latest stages of the disease, when neurons are dying in large numbers.

Do rising brain levels of a plaque-forming substance mean patients are making more of it or that they can no longer clear it from their brains as effectively? … Clearance is impaired in Alzheimer’s disease. We compared a group of 12 patients with early Alzheimer’s disease to 12 age-matched and cognitively normal subjects. Both groups produced amyloid-beta (a-beta) at the same average rate, but there’s an average drop of about 30 percent in the clearance rates of the group with Alzheimer’s. … Scientists calculate this week [that] it would take 10 years for this decrease in clearance to cause a build-up of a-beta equal to those seen in the brains of Alzheimer’s patients. The results have important implications for both diagnosis and treatment.

Here is a more recent paper that reviews what is known of the role of the choroid plexus:

Pathological Alteration in the Choroid Plexus of Alzheimer’s Disease: Implication for New Therapy Approaches

In the recent years, much attention has been directed to the roles of the choroid plexus in the central nervous system (CNS) under both normal and pathological conditions. This specialized ventricular structure has recently emerged as a key player in a variety of processes that monitor and maintain the biochemical and cellular homeostasis of the CNS.

The main role of the choroid plexus is to produce cerebrospinal fluid (CSF) and to maintain the extracellular environment of the brain by monitoring the chemical exchange between the CSF and the brain tissue. This involves the surveying of the chemical and immunological status of the extracellular fluid and the removal of toxic substances as well as important roles in the regenerative processes following traumatic events. In addition to CSF, the plexus produces various peptides which can have nourishing and neuroprotective properties.

…

Morphological alterations of choroid plexus in Alzheimer’s disease (AD) have been extensively investigated. These changes include epithelial atrophy, thickening of the basement membrane, and stroma fibrosis. As a result, synthesis, secretory, and transportation functions are significantly altered resulting in decreased cerebrospinal fluid (CSF) turnover. Recent studies discuss the potential impacts of these changes, including the possibility of reduced resistance to stress insults and slow clearance of toxic compounds from CSF with specific reference to the amyloid peptide.

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